Conventionally, there is a method using screen printing as a typical method for forming light receiving face side electrodes of a solar battery. In the method using screen printing, a print paste is printed in a predetermined location on a silicon substrate using a print masking sheet (screen plate) having a permeable portion of a desired pattern, and is subjected to high-temperature treatment in a baking furnace to form light receiving face side electrodes.
Specifically, stainless-steel wires woven into a net, which is referred to as a screen mesh, are first stretched over a printing plate frame, pulled in the four directions tautly, and fixed thereon. A printing film is then made on the screen mesh, and meshes other than necessary picture lines (pattern of the permeable portion) are closed to form a screen plate. The formed screen plate is set in a screen printer. Further, a silicon substrate is set up with being registered on the screen plate.
A print paste is then put on the screen plate, and pressed and spread over the pattern of the permeable portion. Subsequently, a gumlike blade, which is referred to as a squeegee, is moved while pressurizing an inner printing film of the screen plate. In this way, the print paste permeates a screen mesh (pattern of the permeable portion) of a portion where the printing film is not formed, and is extruded on the silicon substrate positioned under the screen plate and adheres to the silicon substrate to form the desired pattern. After that, the print paste is dried and then baked. In this manner, light receiving face side electrodes of the desired pattern are formed. Since the printing plate can be used to easily form the pattern of the electrodes, this method is now used most widely. As the dimension of a pattern formed by such a method using the screen printing, about 100 μm to 200 μm in line width and about 10 μm to 20 μm in thickness are typical values.
On the other hand, rapid spread of a silicon solar battery to be expected hereafter causes concerns of shortage of silicon row materials. As a countermeasure to this, the power generation efficiency of the solar battery is improved to generate a larger amount of electric power from even the same amount of raw materials as in the conventional manner, and a price per electric power generation of the solar battery is decreased. Therefore, the number of production can be increased. There is a standard in which sizes of a substrate used for the silicon solar battery are standardized, and at present, 156 mm×156 mm is generally used. The improvement of power generation efficiency per substrate improves the power generation efficiency of the solar battery.
As one of methods for improving power generation efficiency, there is, for example, a method in which the wider substantial area of a light receiving face contributing to electric power generation on the substrate is ensured to increase the amount of a current obtained from one substrate. In general, as the area of the light receiving face is increased, the amount of currents generated in the solar battery is increased. On the other hand, the solar battery requires electrodes for collecting the generated currents and allowing the currents to pass therethrough. The electrodes have to be provided on a light receiving face side as long as any special methods are not used. For this reason, the electrodes become a barrier which shields the light receiving face. Accordingly, even in the case of electrodes which allow the current generated in the substrate to pass therethrough, it is necessary that any material shielding the light receiving face be formed so as to minimize its area, and that the area of a region contributing to electric power generation in the light receiving face be maximized, thereby to maximize the current to be obtained.
When grid electrodes having a narrow line width are formed by the conventional screen printing, a screen mesh is easy to be clogged with the print paste. In order to prevent this clogging, a print thickness has to be decreased. As a result, the cross-sectional area of the grid electrodes is decreased, and the electric resistance of the grid electrodes themselves is increased. Therefore, a large amount of current obtained in the substrate can not lead to increase of the power generation efficiency, and the output characteristics of the solar battery cannot be improved.
Furthermore, in the conventional screen printing, it has been necessary that an ink paste permeates the screen mesh. It has been very difficult that a single operation of printing makes the aspect ratio (electrode thickness/electrode width) of an electrode to be 0.3 or larger for the reason of securing a certain fluidity of the ink paste. To this end, the screen printing has been recently able to form an electrode having a line width of 80 μm, for example, but the aspect ratio has to be a value smaller than 0.3, for example, about 0.25, so that it is not able to realize making an electrode to have a narrower line width and a high aspect ratio together. When the aspect ratio of an electrode is equal to or smaller than 0.3, breaking probability is increased due to decrease in electrode thickness, or electrode resistance is increased due to decrease of a cross-sectional area of the electrode, thereby making it impossible to function as an appropriate electrode.
In order to form an electrode pattern having a large thickness, multiple overprints are required, and therefore improvement of release properties of a print paste from a screen plate as well as a print paste capable of being used for the multiple overprints is required. That is, in order to simultaneously make the line width of a grid electrode narrower and the film thickness of the electrode larger using the screen printing, an ink paste capable of multiple overprints is required. In addition, it is necessary that fluidity of the paste be limited and plate release properties from a screen plate be improved. Besides, this method complicates a print process, causes increase in used materials, rise in price, and increase in necessary time, and increases manufacturing cost greatly. Moreover, even if this method is used, due to characteristics of technique for extruding an ink paste from segments of picture lines provided in a screen plate to form a pattern, the cross section of an electrode becomes a barrel-vaulted shape spreading toward the bottom, so that an area which shields a light receiving face is increased. This leads to decrease in photoelectric conversion efficiency.
In order to form a grid electrode having a thickness satisfying the desired characteristics and a narrow line width, it is necessary that based on full knowledge of complexly interrelating characteristics such as a screen printer and printing conditions thereof, a screen plate and specifications thereof, and a print paste for screen printing, process conditions suitable for these characteristics be set. However, in fact, products used for manufacture of solar batteries, which are marketed from apparatus makers and material makers, are purchased and used to perform the manufacturing. Thus, the above-described process conditions are not followed, and a grid electrode having a desired shape cannot be formed. That is, the formation of an electrode by the screen printing has a limit.
On the other hand, there is a method utilizing pattern transfer as a method for forming a predetermined electrode pattern. Conventionally, photolithography for the manufacture of ICs and LSIs has been known as the pattern transfer. Recently, a method for pressing an original intaglio plate onto a substrate is tried in association with simplification of manufacture processes and cost reduction. For example, Patent Literature 1 describes a method for simply and economically performing high-precision pattern transfer utilizing light curing type nanoimprint.